1. Field of the Invention
This invention relates to an optical sensor for measuring physical magnitudes.
More specifically, the invention deals with a sensor involving optical phenomena for measuring physical magnitudes, essentially though not exclusively a pressure, and in particular, the modulation frequency of a pressure.
2. Description of the Prior Art
There are a number of cases where it is desirable to measure the modulation frequency of a pressure, specifically of a differential pressure without the need for a measurement of the differential pressure value in itself.
This is particularly the case for gas eddy counters.
This type of counter, based on the Karman effect, is well known. A detailed description thereof can be found in French Pat. No. 2,357,868. It will be recalled that such a counter comprises an obstacle provided across the conduit inside which flows the fluid whose flow rate is to be measured. This obstacle generates eddies alternately breaking off from both sides of the obstacle which are positioned substantially parallel to the fluid flow. The generation frequency of such eddies is proportional to the fluid velocity.
To measure this frequency, the changes in the fluid pressure difference in the vicinity of both sides of the obstacle can be measured. In order to do so, the obstacle comprises two conduits which open at both sides of the obstacle. It should be noted that in such a case, the characteristic magnitude is the differential pressure modulation frequency and not the actual differential pressure.
In order to measure the differential pressure modulation frequency, it has already been proposed to use a sensor comprising a membrane on the faces of which are respectively applied both pressures differing by an amount to be measured. The membrane's displacement is measured by a variable inductance detector.
Such a detector, which implies that an electric current must flow, is not satisfactory in the case where the eddy counter is used for measuring the flow rate of a flammable gas, since it then requires the use of an explosion-proof casing.
Moreover, the distance between the place where the counter is installed with its differential pressure sensor and the monitoring facility where it is desired to have a display of the flow rate measurement, is often significant. In the case of the inductive sensor, this information is transmitted as an electric signal which may be disturbed by electromagnetic interference.
The same problem can occur when it is desired to make a remote measurement of a static pressure or of other physical magnitudes such a force and an acceleration, the vibration frequency of a piece, and the like.
To overcome these drawbacks, one object of the invention is to provide a sensor for measuring a physical magnitude, such as, in particular, the modulation frequency of a pressure, which requires no electric or electronic circuitry in the close vicinity of a counter and moreover ensures that the transmission of the measurement signal to a remote location is not liable to be perturbed by electromagnetic interference.